The performance criteria for lubricants such as those used in the crankcase of diesel and spark ignition engines may become increasingly more severe as users require lubricants with longer useful lives. For this and other reasons, the efficiency and useful lives of oil-based lubricants, particularly crankcase lubricants, must be improved.
Oxidation of the oil component in the lubricant substantially shortens its useful life. Oxidation yields deposit precursors, corrosive acids, and an undesirable increase in viscosity. While high quality basestocks tend to be relatively resistant to oxidation, contaminants (e.g., iron) and common additives can greatly accelerate oxidation. Inclusion of dispersants (e.g., polyamine or polyester derivatives of alkenyl succinic acids or anhydrides) is desirable for oil performance, but these additives may also be oxidized in the oil, which is undesirable; and in any event experience has shown that the effectiveness of dispersants decreases with time, probably due to degradation of the dispersant.
Despite the great volume of research directed toward improving the useful life of lubricants, particularly crankcase lubricants, there remains a need for improving the dispersancy retention capability of crankcase lubricants.